The invention originates from the field of reactive power compensation and voltage control in an AC power transmission or distribution system, in particular from the field of shunt compensation, where the following is known in the art. In “FACTS Technology for Reactive Power Compensation and System Control” by H. K. Tyll, paper for IEEE/PES panel session on FACTS, Nov. 8-11, 2004, Sao Paulo, different types of solutions for shunt compensations are shown. On the one hand there are mechanically-switched capacitors (MSC) and mechanically switched reactors (MSR), which are cost-effective solutions providing steady-state voltage support and being characterized by a slow response time and a discontinuous control working at a reduced precision level. As a disadvantage, high transient currents and voltages may occur during switching operations. On the other hand there are solutions which are based on power electronic switching, such as shunt-connected static var compensators (SVC) and static synchronous compensators (STATCOM), which provide dynamic voltage support and are characterized by a fast response time and a control which is performed continuously and at a higher precision level. Here the main disadvantage is the occurrence of commutation losses.
With regards to MSCs, the following is further known from “CAPS: Improving power system stability using the time-overvoltage capability of large shunt capacitor banks”, by C. W. Taylor and A. L. Van Leuven, IEEE Transactions on Power Delivery, Vol. 11, No. 2, April 1996. In the article, an arrangement for reactive shunt compensation, called CAPS, is described where during low voltage emergencies several series groups of wye-connected capacitor banks are shorted to increase reactive power output. The CAPS equipment includes as main components a vacuum breaker and a current limiting reactor which together perform the switching action as well as a magnetic potential transformer in order to discharge trapped charges. The CAPS (capacitor bank series group shorting) arrangement was used to create one single high voltage step in emergency situations, namely when the voltage dropped below a threshold for twelve seconds. In this case, the several groups of capacitor banks were short-circuited and thereby the reactive power output was increased by 27%. Due to the use of a mechanical switch, the response time of the arrangement was considerably low. For example, in the article it is given a time constant of 140 ms for opening the vacuum breaker.
It has been recognized by the inventors that in practice a continuous reactive power compensation and control as provided by SVCs and STATCOMs is required only for a limited number of medium voltage or high voltage AC systems, so that it is often not economically justified to install these high level systems in all places where such compensation and control might be needed only occasionally. The employment of MSCs, however, does not always offer the flexibility required, due to their large step response time as well as to unavoidable transients.